For anodizing, metallic hollow bodies or workpieces are secured on projecting fingers of electrically conductive workpiece holders, and the workpiece holders are transferred in steps in a predefined work cycle through a series of treatment baths including at least one anodizing bath in which anodic oxidation on the surfaces of the cup-shaped workpieces is effected by a closed electric circuit between the workpiece holder and a cathode in the treatment bath.
Anodizing or anodic oxidation is an electrochemical process that converts the metal surface of a workpiece into a metal oxide. The resulting oxide layer is intimately bonded to the metallic base material. The layer thickness can be set in a defined manner by the selection of appropriate process parameters. Anodizing is primarily used for surface finishing workpieces of aluminum or aluminum alloys. The anodized layer durably protects the aluminum from environmental influences, is easy to clean and, through the structure of the oxide layer, allows decorative coloration. This method is used in practice to provide cup-shaped workpieces such as sleeves caps with a high-quality decorative surface.
In addition to an anodizing bath, the treatment baths comprise further baths in which the workpieces are degreased, chemically treated, rinsed and sealed. The color effect and the gloss effect can be achieved and influenced by different immersion times in the different treatment liquids.
In a well known method, the workpiece holders that were previously loaded with the workpieces to be treated are oriented pointing downward from a conveyor, and the workpiece are transported upright and are vertically dipped into the treatment liquid of the treatment baths. In this method, workpieces at the lower end of the workpiece holder are in the treatment liquid for a longer time than the workpieces at the upper end of the workpiece holder. The different dwell time in the anodizing bath and in a downstream coloring bath has a negative effect on the quality of the workpiece surfaces. The anodized and colored surfaces of the workpieces differ, for example, with regard to color intensity that greatly depends on the dwell time of the workpieces in the coloring bath.
Moreover, the orientation of the workpieces results in quality differences between the anodized surfaces on the upper region and lower region of the hollow body. Another disadvantage of the known method is that after pulling the vertically oriented workpiece holder out of the treatment bath, the treatment liquid does not completely flow off the workpieces within the limited time determined by the work cycle. This results in significant carryover losses of the treatment liquid during movement of the workpiece holder from bath to bath. This has a negative effect, among other things, on the sewage disposal and the consumption of chemicals.
In a method known from U.S. 2008/0257717 (U.S. Pat. No. 8,293,077), the workpiece holders that were previously loaded with the cup-shaped workpieces to be treated are transferred to a drum that rotates in steps in the treatment bath. Here, the workpiece holders move through the treatment bath on a helical path. Depending on the needed dwell time, the drum performs one or more full revolutions. A plurality of treatment baths that are equipped with a suitable device are in a row one behind the other such that the workpiece holders with the workpieces to be treated can be guided through a plurality of different treatment baths. The dwell time in the treatment baths depends on the number of revolutions of the drum and can be varied only in steps having a time that is equal to a whole-number multiple of the time of a full revolution. It is difficult to set a very short treatment time and to combine it with baths that require a very long dwell time. If the drum needs a plurality of revolutions for setting a long dwell time, it is also disadvantageous that during the treatment in the treatment liquid, the workpieces leave the treatment liquid again and again and dip again into the treatment liquid. The method-related limitations have a negative effect on the surface quality of the workpieces.
In a method of anodizing cup-shaped workpieces known from DE 12 74 979, the workpieces are fixed on an endless titanium or zirconium belt that forms loops and is guided through a row of treatment baths. The dwell time of the workpieces can be varied by the number of belt loops within a treatment bath. In this method too, a significant carryover of the treatment liquid from treatment bath to treatment bath is unavoidable.